This invention relates to effluent treatment.
Sodium hydroxide is used in many industries for cleaning and extracting impurities for intermediate and final products. Examples include:
(i) caustic scouring of cotton fibre and fabric; PA0 (ii) bottle washing; PA0 (iii) peeling of fruit and vegetables; PA0 (iv) treating the fibres for the paper industry. PA0 neutralisation PA0 biological oxidation PA0 ion exchange PA0 evaporation PA0 direct discharge to the environment. PA0 (i) if the solution has a pH greater than 9, bringing the pH of the solution to a pH in the range 7 to 9, PA0 (ii) filtering the solution from step (i) to remove suspended insoluble matter having a size greater than 0.1 microns; PA0 (iii) filtering the filtrate from step (ii) to remove multivalent ions and organic matter having a molecular mass greater than about 300 daltons; PA0 (iv) providing an electrochemical cell in which an anode compartment is separated from a cathode compartment by a cation selective membrane; PA0 (v) passing the filtrate from step (iii) into the anode compartment of the electrochemical cell; and PA0 (vi) passing an electrical direct current through the cell to cause alkali metal ions to pass through the cation selective membrane and alkali metal hydroxide to be produced in the cathode compartment and an acidic component to be produced in the anode compartment. PA0 (i) contacting the organic material with the hydroxide solution to produce an effluent comprising an alkali metal hydroxide solution which contains multivalent ions and soluble and insoluble organic and inorganic matter; PA0 (ii) reducing the pH of the effluent to a value in the range 7 to 9 by contacting it with an acid gas; PA0 (iii) filtering the solution from step (ii) to remove suspended insoluble matter having a size greater than 0,1 microns. PA0 (iv) filtering the filtrate from step (iii) to remove multivalent ions and organic matter having a molecular mass greater than about 300 daltons; PA0 (v) providing an electrochemical cell in which an anode compartment is separated from a cathode compartment by a cation selective membrane; PA0 (vi) passing the filtrate from step (iv) into the anode compartment of the electrochemical cell; PA0 (vii) passing an electrical current through the cell causing alkali metal ions to pass through the cation selective membrane and alkali metal hydroxide to be produced in the cathode compartment and the acid gas to be produced in the anode compartment; PA0 (viii) returning the alkali metal hydroxide to step (i); PA0 (ix) using the acid gas produced in the anode compartment for reducing the pH of the effluent in step (ii). PA0 (x) returning the depleted solution from the anode compartment, if required, to step (i).
Specialised surfactants may be used to prevent the precipitation and redeposition of calcium and magnesium salts in the effluents from these processes. The amount of sodium hydroxide which is chemically consumed in the washing/extraction process is usually small. the caustic is then washed from the cleaned article and dilute caustic effluent containing organic and inorganic impurities is produced. The treatment of this effluent is problematic in that it has a high pH value and is usually high in organic content.
Typical methods of treatment include:
Except for evaporation, the other methods of treatment do not recover the caustic and result in an alkaline or saline effluent. In South Africa and elsewhere saline effluents are particularly problematic in that the increasing salinity of fresh water environments is a major cause of decreasing water quality which thus reduces its potential for reuse.